DE102007014399B4 - Control loop with two operating modes for pulsed current transformers - Google Patents

Control loop with two operating modes for pulsed current transformers

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Publication number
DE102007014399B4
DE102007014399B4 DE200710014399 DE102007014399A DE102007014399B4 DE 102007014399 B4 DE102007014399 B4 DE 102007014399B4 DE 200710014399 DE200710014399 DE 200710014399 DE 102007014399 A DE102007014399 A DE 102007014399A DE 102007014399 B4 DE102007014399 B4 DE 102007014399B4
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Prior art keywords
voltage
reference voltage
current
output
error amplifier
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DE200710014399
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German (de)
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DE102007014399A1 (en
Inventor
Hans Schmeller
Christophe Vaucourt
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Texas Instruments Deutschland GmbH
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Texas Instruments Deutschland GmbH
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00Conversion of dc power input into dc power output
    • H02M3/02Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators

Abstract

DC-DC converter, comprising
a power stage (NM1) driven by a pulse width modulator (PWM),
a first error amplifier (A1) having a first input coupled to a first reference voltage (VREF1) and a second input connected to a current sink (CS1) through which a current flows from an output (VOUT) of the power stage (NM1 ) is coupled to receive a first feedback voltage (FB1),
a second error amplifier (A2) having a first input coupled to a second reference voltage (VREF2) and a second input coupled to the output (VOUT) of the power stage (NM1) to provide a second feedback voltage (FB2) receive,
switching means (SW1) for connecting a control input (IN) of the pulse width modulator (PWM) to an output of the first error amplifier (A1) in a current control mode and to an output of the second error amplifier (A2) in a voltage regulation mode, wherein a reference voltage control means is arranged that the value of the second reference voltage (VREF2) after a transition from the ...

Description

  • The present invention relates to a DC-DC converter, more specifically to a DC-DC converter having a power stage driven by a pulse width modulator.
  • DC-DC converters are used for a wide variety of applications and devices, especially for battery powered handheld and portable applications. Some devices require several different voltages or currents for different parts and functions of the device. An example of such a device is a mobile phone with an integrated camera module in which for the normal mobile phone functions a constant voltage and for the flash of the camera, a high current may be needed. One conventional approach provides a single single loop DC-DC converter stage for each function. However, this approach is expensive, power consuming and unwieldy.
  • The US 6456051 B2 as well as the EP 0 752 748 A1 each shows a DC-DC converter comprising a power stage driven by a pulse width modulator. In addition, the DC-DC converter comprises a first and a second error amplifier. The first error amplifier has an input coupled to a first reference voltage source. In addition, the first error amplifier has a second input coupled to a current sink through which a current is conducted from an output of the power stage to receive a first feedback voltage. The second error amplifier has a first input coupled to a second reference voltage source. In addition, the second error amplifier has a second input coupled to the output of the power stage to receive a second feedback voltage. The DC-DC converter further comprises switching means for connecting a control input of the pulse width modulator to an output of the first error amplifier in a current control mode and to an output of the second error amplifier in a voltage regulation mode.
  • US 7106037 B2 also shows a DC / DC voltage converter that can toggle between a current and a voltage regulation mode, with the reference voltage in the loop being reduced at an error amplifier to prevent the switching of the output current when no load current is required.
  • However, the DC / DC voltage converters according to the prior art are not adapted to enable a smooth transition between the operating modes in a simple manner with little circuit complexity.
  • It is an object of the present invention to provide a DC-DC converter which is suitable for many different applications and yet smaller and less complex than prior art solutions.
  • Accordingly, there is provided a DC-DC converter having a pulse width modulator driven power stage including a first error amplifier having a first input coupled to a first reference voltage source and a second input connected to a current sink through which a current of one Output of the power stage is coupled to receive a first feedback voltage, a second error amplifier having a first input coupled to a second reference voltage source, and a second input coupled to the output of the power stage to a second reference voltage and a switching means for connecting a control input of the pulse width modulator to an output of the first error amplifier in a current control mode and having an output of the second error amplifier in a voltage regulation mode. Thus, the DC-DC converter according to the present invention can support at least two different control mechanisms. Each of the control mechanisms can be implemented by a single control loop (feedback loop) coupled to a single error amplifier. A control input to the pulse width modulator is coupled between the outputs of the error amplifiers, thereby shifting the DC-DC converter from one control mode (eg, to regulate the output voltage to a constant level) to another control mode (eg, to provide a constant output current ) is switched.
  • However, when using the DC-DC converter for two different applications of different sizes (i.e., output voltages or currents), the switching can trigger strong loop responses. This can lead to unwanted and unstable output signals of the DC-DC converter.
  • Therefore, in the preferred embodiment of the present invention, the DC-DC converter includes reference voltage control means for temporarily setting the value of the second reference voltage source to reduce the transients when switching between the voltage and current control modes. This can be implemented by setting the reference voltage control means such that it changes the value of the second reference voltage source after a transition from the voltage regulation mode to the current regulation mode from a predetermined reference voltage value to the first reference voltage and supplies the first comparison result to the input of the pulse width modulator via the first switch only if the second reference voltage is substantially equal to the first reference voltage Reference voltage is. The reference voltage control loop is further configured to hold the second reference voltage close to the second reference voltage during the current control mode. Finally, the reference voltage control means is arranged to change the second reference voltage back to the predetermined reference voltage value only after the output of the second error amplifier has been connected to the control input of the pulse width modulator via the first switch in response to a transition from the current control mode to the voltage regulation mode. Accordingly, the present invention provides a means for preventing the destabilization of the DC-DC converter due to transients caused by the switching between different control modes. The DC-DC converter according to the present invention is adapted to switch quickly and smoothly from the current control mode to the voltage regulation mode and vice versa. When the DC-DC converter controls the output voltage in the voltage regulation mode and the DC-DC converter is switched from the voltage regulation mode to the current regulation mode, the second reference voltage is continuously changed so as to approach the first reference voltage before the first switch Output of the first error amplifier connects to the pulse width modulator. By continuously adjusting the second reference voltage, the output voltage at the output node of the DC-DC converter is also steadily adjusted, and the first feedback voltage approaches the first reference voltage. The first and second reference voltages are at substantially the same voltage level when the control input of the pulse width modulator is switched from the output of the second error amplifier to the first error amplifier. Accordingly, oscillations of the voltages and the currents due to a sudden change of the input signal of the first control loop (ie, the feedback loop) are minimized, and the DC-DC converter quickly oscillates in the current control mode. During the current control mode, the second reference voltage is maintained at a voltage level that is basically equal to the voltage level appearing in the current feedback mode at the second feedback path. Accordingly, the two input signals of the second error amplifier are the same. When the DC-DC converter is switched back to the voltage regulation mode, no transients occur in the second control loop (or feedback loop). Finally, to generate the correct output voltage at the output node of the DC-DC converter, the second reference voltage is steadily returned to an initial and predetermined value that is set to produce a particular output voltage.
  • Other aspects of the present invention will become apparent from the following description of the preferred embodiments with reference to the accompanying drawings. Show it:
  • 1 a simplified circuit diagram of a DC-DC converter according to the present invention,
  • 2 temporal waveforms of voltages and currents of a DC-DC converter without reduction of the transient processes,
  • 3 a simplified circuit diagram of a circuit for setting the second reference voltage according to the present invention,
  • 4 temporal waveforms of voltages and currents of a DC-DC converter with a circuit according to 3 which is switched from voltage regulation mode to current regulation mode, and
  • 5 temporal waveforms of voltages and currents of a DC-DC converter with a circuit according to 3 , which is switched from current control mode to voltage regulation mode.
  • 1 shows a simplified circuit diagram of a DC-DC converter according to an aspect of the present invention. The DC-DC converter includes a power switch such as a power NMOS transistor NM1 and a second switch (or a diode) PM1 and a pulse width modulator PWM for controlling the power switch NM1 and the switch PM1. The primary power supply is represented by VIN on one side of the inductor L. The pulse width modulator PWM switches the switches NM1 and PM1 to provide a regulated output voltage VOUT at the output node of the DC-DC converter. A capacitor C1 is used for buffering and smoothing the output voltage VOUT. The DC-DC converter provides two modes of operation. A current control mode and a voltage regulation mode. In the current control mode, a load (for example, an LED) goes through the current sink CS1 is supplied with a substantially constant output current, and the voltage VLED is regulated to a certain value. The output voltage VOUT basically follows the voltage VLED plus the LED forward voltage, which typically means that VOUT decreases. In the voltage regulation mode, a constant output voltage VOUT is generated at the output terminal. Since the output current path is blocked by the LED via SW3, VLED basically follows VOUT. There are two error amplifiers A1, A2 for comparing the feedback voltages FB1, FB2 with a corresponding reference voltage VREF1, VREF2. A first feedback path is coupled to the first error amplifier A1 and is arranged to provide a first feedback voltage FB1 derived from the current source CS1 which provides the output current ILED flowing through the LED. In the in 1 In the embodiment shown FB1 is equal to VLED. The error amplifier A1 compares the first feedback voltage FB1 with a first reference voltage VREF1 to provide a first comparison result at the output of the first error amplifier A1. A second feedback path is coupled to the second error amplifier A2 and configured to provide a second feedback voltage FB2 derived by the voltage divider R1, R2 from the output voltage VOUT. The feedback voltages FB1, FB2 are different from each other. The second error amplifier A2 compares the second feedback voltage FB2 with a second reference voltage VREF2 to provide a second comparison result. The first switch SW1 is arranged to selectively switch the control input of the pulse width modulator PWM to the output of the first error amplifier A1 or to the output of the second error amplifier A2 in response to a transition from the current control mode to the voltage regulation mode. A switch SW3 is used to connect the LED to the current sink CS1, thereby turning on the LED. The current sink CS1 may be implemented as a current mirror or the like. To establish a predetermined, controlled VLED, the DC-DC converter enters the current control mode. In the current control mode, the voltage FB1 (VLED) at the current sink CS1 is used as the input to the first error amplifier A1. When the output of the error amplifier A1 is switched to the control input of the pulse width modulator, the first control loop is closed and the output voltage VOUT follows the regulation for VLED. The LED merely serves to represent a light-emitting semiconductor component. Instead of an LED, several LEDs can be used in parallel or in series as well as a single or several LEDs on a chip. A switching sequence control SC symbolizes additional control mechanisms that are used to provide the correct switching sequence for the switches SW1 and SW3. The two error amplifiers A1 and A2 share the same compensation network (represented here by C2). Depending on the control mode, either the first error amplifier A1 or the second error amplifier A2 is connected to the compensation capacitor.
  • The dashed line in 1 indicates a possible partitioning of components typically integrated on a single die silicon or other semiconductor substrate. The components outside the dashed line are preferably not integrated, ie they are external and are typically located together with the chip on a circuit board. The dashed line in 1 The configuration shown concerns a typical CMOS or BICMOS technology. However, a different layout may be used for other technologies.
  • 2 shows time waveforms relating to the output voltage VOUT, the voltage VLED at the light emitting diode, the currents IL through the inductance L and the current ILED through the light emitting diode LED refer (see. 1 ). 2 shows a typical mode transition situation. The DC-DC converter is switched from the voltage regulation mode to the current regulation mode at 200 μs. Accordingly, the DC-DC converter has been switched from the control of VOUT, ie, the voltage regulation mode, to the regulation of VLED, ie, the current control mode. Thus, the DC-DC converter allows switching from one control mechanism to another by switching the control input of the pulse width modulator between the outputs of the first and second error amplifiers.
  • However, when the DC-DC converter switches from voltage regulation to current regulation, the voltage at the light emitting diode VLED is initially at an extremely high voltage level far away from the voltage level to be reached in a current control mode. During the voltage regulation mode, the light emitting diode device LED is disconnected by the switch SW3, which means that VLED is nearly equal to VOUT, which is typically 5V, for example. The current control loop (the first feedback loop) attempts to slew VLED down to a much lower voltage, for example, 250 mV. 2 (a) shows the input voltage VIN from the primary power supply constant for the entire time and the output voltage VOUT which drops after 200 μs and oscillates for the next 40 μs. 2 B) shows the voltage VLED at the Led. The first part of the waveform is not shown because VLED is close to VOUT as described above. VLED drops to 200 μs after switching from the voltage control mode to the current control mode. As in 2 (c) As shown, the control activities of the first feedback loop cause significant unregulated negative inductance currents. This can cause a voltage overshoot in the primary power supply (eg a battery). 2 (d) shows the current through the light emitting diode LED. After switching on the switch SW3 (in 1 shown), the current ILED increases to reach a final value of approximately 0.5A. As in all 2 (a) - (d), the transition from the voltage regulation mode to the current regulation mode entails a long period in which currents and voltages approach their final values in an oscillatory manner.
  • 3 shows a simplified circuit diagram of a circuit which is used to set the second reference voltage VREF2 according to the present invention. During the current control mode, however, the second feedback voltage FB2 is coupled to the transistor NM2 and the switch SW2 to the gate of the transistor NM3. The current sources CS2 and CS5 receive and sink approximately the same current (ICS5 is equal to ICS2) flowing through the transistor NM2. Since the transistor NM2 is diode-coupled and the current ICS6 provided by the current source CS6 is also equal to ICS5 and ICS2, the gate-source voltage of the transistor NM3 reaches such a value that VREF2 is equal to FB2. When the control mode of the DC-DC converter is switched from the current control mode to the voltage regulation mode, the switch SW3 is turned on and the current source CS3 adds a certain current ICS3, so that the voltage at C3 and at the gate of the transistor NM3 steadily increases. Accordingly, the voltage VREF2 approaches VREFIN. When the DC-DC converter is switched from the voltage regulation mode to the current regulation mode, the switch SW4 is turned on, and the switch SW3 is turned off. Accordingly, the second reference voltage VREF2 is slowly shut down. The converter remains equal to VREF1 in the voltage regulation mode until VREF1 (eg, 250 mV), and then the DC-DC converter is switched to the current regulation mode, the comparison between VREF1 and VREF2 is performed by the error amplifier A3. The end of the transition is indicated by the signal ET. The switching control of the switches SW2, SW3 and SW4 is symbolized by the sequence control stage SC.
  • 4 shows time waveforms of some voltages and currents in a DC-DC converter according to the present invention with a like in 3 shown reference voltage control means. Consequently, no negative currents occur at the inductance L, and the inductance current IL is continuously regulated to the target value. At 400 μs, the DC-DC converter is switched from the voltage regulation mode to the current regulation mode. As in 4 (a) 1, the reference voltage VREF2 is steadily reduced from about 1.25V to 0.25V, which is the value of VREF1 (in 4 (a) not shown). 4 (b) shows the end-of-transition signal ET indicating that the second reference voltage VREF2 is sufficiently close to the first reference voltage VREF1 so that the control input of the pulse width modulator can be switched. 4 (c) shows the output voltage VOUT and the voltage at the light emitting diode VLED. The output voltage VOUT steadily decreases after 400 μs. VLED shows a sharp edge due to the turning on of the current path through the LED. After this first drop, the voltage VLED steadily approaches 250 mV. 4 (d) shows the currents IL and ILED. Both currents reach their final levels steadily without undue oscillation.
  • 5 shows the same signals as in 4 but now for a change from the current control mode to the voltage regulation mode at 800 μs. In this situation, the output of the second error amplifier is immediately connected to the control input of the pulse width modulator. Since the second reference voltage VREF2 was kept close to the second feedback voltage FB2 during the current control mode, no oscillation occurs. After 800 μs, the second reference voltage VREF2 is steadily increased and reaches approximately 1.25 V after 950 μs. The end-of-transition signal ET changes abruptly at 800 μs, indicating that the control input of the pulse width modulator immediately responds to the output of the pulse width modulator second error amplifier is switched. Since VREF2 is steadily increasing, VOUT follows as in 5 (c) and steadily increases from 3.5 V at 800 μs to 5 V at 950 μs. The in 5 (d) current through the light emitting diode ILED is turned off to 0A, and the current IL through the inductance L enters a periodic oscillation required to produce a constant output voltage VOUT. In general, shows 5 the smooth transition in switching from the current control mode to the voltage regulation mode. VREF2 in 5 (a) reflects the reference voltage setting. The current control mode is executed before 800 μs. When the DC-DC converter is switched to the voltage regulation mode at 800 μs, the internal reference voltage VREF2 starts to ramp up slowly, allowing the control loop to ramp up to follow VREF2 and thus VOUT without any significant influence on the inductor current IL is raised to 5V.

Claims (1)

  1. DC-DC converter, comprising a power stage (NM1) driven by a pulse width modulator (PWM), a first error amplifier (A1) having a first input coupled to a first reference voltage (VREF1) and a second input connected to a current sink (CS1) through which a current flows from an output (VOUT) of the power stage (NM1 ) is coupled to receive a first feedback voltage (FB1), a second error amplifier (A2) having a first input coupled to a second reference voltage (VREF2) and a second input coupled to the output (VOUT) of the power stage (NM1) to provide a second feedback voltage (FB2) receive, switching means (SW1) for connecting a control input (IN) of the pulse width modulator (PWM) to an output of the first error amplifier (A1) in a current control mode and to an output of the second error amplifier (A2) in a voltage regulation mode, wherein a reference voltage control means is arranged that it changes the value of the second reference voltage (VREF2) after a transition from the voltage regulation mode to the current regulation mode from a predetermined reference voltage value to the first reference voltage (VREF1) and the output of the first error amplifier (A1) to the input of the pulse width modulator via the switching means ( SW1) is coupled only when the second reference voltage (VREF2) is substantially equal to the first reference voltage VREF1), wherein during the current control mode the second reference voltage (VREF2) is maintained at a voltage level substantially equal to that in the current reference current mode on the second feedback path is to be changed back to the predetermined reference voltage value by the second reference voltage only after the output of the second error amplifier (A2) via the switching means (SW1) in response to a transition from the voltage control mode in the Stromregelungsbetriebsart with the Control input (IN) of the pulse width modulator (PWM) has been connected.
DE200710014399 2007-03-26 2007-03-26 Control loop with two operating modes for pulsed current transformers Active DE102007014399B4 (en)

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DE200710014399 DE102007014399B4 (en) 2007-03-26 2007-03-26 Control loop with two operating modes for pulsed current transformers
US12/056,046 US8044649B2 (en) 2007-03-26 2008-03-26 Dual mode regulation loop for switch mode power converter

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US8044649B2 (en) 2011-10-25
US20080238387A1 (en) 2008-10-02
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